A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In device manufacturing methods using a lithographic apparatus, the minimum feature size, often referred to as the critical dimension (CD), is determined by the wavelength (λ) of the exposure radiation and the numerical aperture (NA) of the projection system used by the lithographic apparatus. Various techniques have been developed to reduce the critical dimension, and the effects of these techniques are commonly combined into a scaling factor known as k1. The relationship between the scaling factor, the numerical aperture, the wavelength of the radiation used, and the critical dimension is as follows:CD=k1·λ/NA A process to obtain a reduced scaling factor comprises using near field imaging to expose a layer of resist, which involves the generation and use of near-field radiation. In a known method, the near-field radiation is generated using a specifically designed mask which is independent of a resist coated substrate to which a pattern is to be applied. In order to use the near-field radiation to apply a pattern to the resist on the substrate, the mask needs to be accurately spaced apart from the resist across all areas of the resist to be patterned, or alternatively the mask needs to be brought into contact with all areas of the resist to be patterned. In practice, it is difficult to achieve the accurate separation between the mask and the resist that is required to accurately apply patterns to the resist. Alternatively, if the mask is to be brought into contact with the resist, it is difficult to ensure that all areas of the resist to be patterned are actually in contact with the mask, and it is also possible that the mask may damage the resist when it is brought into contact with it.